A substrate is the structure with which an enzyme bonds with, to catalyze a reaction.
An enzyme catalyzes, or speeds up, chemical reactions. They do so by lowering the energy required for the reaction to start. However, they cannot work alone.
To catalyze a reaction, an enzyme has to bind with a substrate. The enzyme has an area called the active site, this is the area which bonds to the substrate. The “Induced Fit” model is often used to explain this. In the model, the active site and substrate do not fit exactly, and thus the enzyme must slightly alter it’s shape to accommodate the active site.
This is also the reason extreme temperatures cause enzymes to denature, or stop functioning; if either substrate or enzyme changes shape, the enzyme cannot catalyze reactions anymore.
Here’s an image to help depict this: